Theory of zeolite supralattices: Se in zeolite Linde type A

We study theoretically properties of Se clusters in zeolites, and choose zeolite Linde type A (LTA) as a prototype system. The geometries of free-space Se clusters are first determined, and we report the energetics and electronic and vibrational properties of these clusters. The work on clusters includes an investigation of the energetics of C3-C1 defect formation in Se rings and chains. The electronic properties of two Se crystalline polymorphs, trigonal Se and -monoclinic Se, are also determined. Electronic and vibrational properties of the zeolite LTA are investigated. Next we investigate the electronic and optical properties of ring-like Se clusters inside the large -cages of LTA. We find that Se clusters inside cages of silaceous LTA have very little interaction with the zeolite, and that the HOMO-LUMO gaps (HOMO standing for highest occupied molecular orbital and LUMO for lowest unoccupied molecular orbital) are nearly those of the isolated cluster. The HOMO-LUMO gaps of Se6, Se8, and Se12 are found to be similar, which makes it difficult to identify them experimentally by absorption spectroscopy. We find that the zeolite/Se8 nanocomposite is lower in energy than the two separated systems. We also investigate two types of infinite chain encapsulated in LTA. Finally, we carry out finite-temperature molecular dynamics simulations for an encapsulated Se12 cluster, which shows cluster melting and formation of nanoscale Se droplets in theα-cages of LTA. (author)